<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .class1{
            background-color: red;
        }
        #app{
            color: blue;
        }
    </style>
</head>
<body>
 <div class="class1" id="app" flag="1" haha="1234">
     hahah
 </div>
</body>
</html>